Match Function Doesn't Find Values

I've been using a Index and Match Functions to locate values,

What I'm trying to do is as follows:

Column A contains x values and Column B contains another value

A length is entered at the top of the worksheet, when the x value is greater than half the length the values will be mirrored about the centre point.

To do this i'm doing x-(length/2) to give the corresponding value of x on the other side of the length

I can do the If functions etc to sort all this out, but the match function isnt working, see attached, it isn't finding values which are in the list, when i use 0 for exact match, i've tried using 1 but it still isn't finding the correct values.